UHMWPE has a molecular weight of at least 1,000,000 Da, which is 10 to 100 times greater than the molecular weight of high-density polyethylene (HDPE). UHMWPE offers major advantages in increased impact resistance, tensile strength, abrasion resistance, and stress-crack resistance. UHMWPE can be produced by Ziegler polymerization. The process requires exceptionally pure ethylene and other raw materials. Like conventional HDPE, UHMWPE made by Ziegler polymerization has a broad molecular weight distribution Mw/Mn (Mw is the weight average molecular weight, Mn is the number average molecular weight) of within the range of 5 to 20.
However, UHMWPE with a narrow molecular weight distribution Mw/Mn of less than 5 have improved mechanical properties. Newly developed metallocene and single-site catalysts advantageously provide polyethylene and other polyolefins with very narrow molecular weight distribution (Mw/Mn from 1 to 5). The narrow molecular weight distribution results in reduced low molecular weight species and higher Mn which further improves abrasion resistance. These new catalysts also significantly enhance incorporation of long-chain α-olefin comonomers into polyethylene, and therefore reduce its density. Unfortunately, however, these catalysts produce polyethylene having a lower molecular weight than that made with Ziegler-Natta catalysts. It is extremely difficult to produce UHMWPE with conventional metallocene or single-site catalysts.
However, some allege to have obtained UHMWPE with various single-site catalysts.
U.S. Pat. No. 7,951,743 discloses an ultra-high molecular weight, linear low density polyethylene obtained with a catalyst system that comprises a bridged indenoindolyl transition metal complex, a non-bridged indenoindolyl transition metal complex, an alumoxane activator and a boron-containing activator. The ultra-high molecular weight, linear low density polyethylene has a Mw greater than 1,000,000 and a density less than 0.940 g/cm3.
U.S. Pat. No. 7,091,272 B2 discloses an olefin polymerization process in the presence of a clay, an activator, and a transition metal complex that has at least one pyridine moiety-containing ligand. The presence of clay increases the catalyst activity. The process is suitable for making ultra-high molecular weight polyethylenes (UHMWPE).
U.S. Pat. No. 6,635,728 B2 discloses an ethylene polymerization process with a supported quinolinoxy-containing single-site catalyst in the presence of a non-alumoxane activator, but in the absence of an α-olefin comonomer, an aromatic solvent, and hydrogen to produce UHMWPE.
US 2010/0056737 A1 discloses a process of manufacturing high, very high, and ultra high molecular weight polymers comprising predominantly ethylene monomers. Ethylene is reacted in the presence of a catalyst system to produce a polymer having a viscosimetrically-determined molecular weight of at least 0.7×106 g/mol. The catalyst system generally includes a bridged metallocene catalyst compound, optionally with a cocatalyst. The catalyst is characterized by a zirconium dichloride central functionality and a dimethyl silandiyl bridge between five-membered rings of indenyl groups. Both rings of the metallocene compound are substituted at the 2-position with respect to the dimethyl silandiyl bridge with a C1-C20 carbonaceous group.
In WO 2011/089017 A1, a novel UHMWPE material is disclosed, comprising both Hf and Cr as a catalyst residue, preferably with the proviso that the Cr catalyst is not comprised in oxidic form in the polyethylene, displaying excellent abrasion resistance amongst other properties. The Hf and Cr, in the context of the invention, stems preferably from single site catalyst of the metallocene and/or half-sandwich metallocene type comprising organic, multidentate ligands (i.e. not from a Phillips catalyst).
WO 2010/139720 A1 pertains to a process for manufacturing a UHMWPE, wherein olefin monomers are contacted with a catalytic system under polymerisation conditions under formation of a polyethylene, wherein the catalytic system comprises an active component on a particulate carrier in a site density in the range of 5*10−9 to 5*10−6 mole of catalytic sites per m2 of carrier surface area, the particulate carrier having an average particle diameter in the range of 1-300 nm, wherein the polyethylene has a Mw of at least 500 000 g/mol, and an elastic shear modulus G0N, determined directly after melting at 160° C. of at most 1.4 MPa.
Fujita et al. at Mitsui Chemicals Inc. disclosed a new class of catalysts for living olefin polymerisations, the so-called phenoxyimine-based (FI) catalysts (Catalysis Today, Volume 66, Issue 1, 15 Mar. 2001, Pages 63-73 and Chemical Review, 2011, 111, 2363-2449). M. S. Weiser et al. report in the Journal of Organometallic Chemistry 2006, 691, 2945-2952 tailoring such a phenoxyimine catalyst for the synthesis of UHMWPE, as well as atactic polypropylene. Macromolecules 2011, 44, 5558-5568, also discloses phenoxyimine catalysts to prepare disentangled UHMWPE namely at conditions of a) low polymerization temperature, so that the crystallization rate is faster than the polymerization rate, and b) low concentrations of active sites, so as to minimize the interaction of the growing chains.
However, a drawback of such phenoxyimine-based catalysts is that the phenoxy group does not provide sufficient rigidity to prevent the resulting metallic complex from adopting different conformations leading to the presence of multiple catalytic sites. Furthermore, phenoxy groups only have a limited number of sites which can bear substituents, these being needed for tailoring and fine-tuning in order to increase catalytic activity and/or enhance the control over the UHMWPE microstructure (short-chain branching, long-chain branching etc).
Thus a new family of single-site catalysts are needed, which have ligands which are more rigid, which are easier to fine-tune with a larger number of possible substituents but are also capable of preparing UHMWPE, preferably having a narrow molecular weight distribution Mw/Mn (also called polydispersity index) namely from 1 to 5, even more preferably 1 to 3.
The solution to this technical problem was found by providing Group 4 transition metal based catalysts having iminonaphthol ligands. The backbone of the iminonaphthol ligand is larger and completely planar because of the use of the aromatic naphthalene skeleton (FIG. 1). Moreover, without being bound by theory, this would lock the conformation of these complexes, unlike the phenoxyimine ligands of the prior art which are known to display flexible coordination modes. This gives opportunities for increased ligand fine-tuning and results in increased rigidity of group 4 transition metal complexes thus providing more precise control over the microstructure of polymers prepared by such catalysts. This broad family of ligands has already been disclosed in WO 2009/133026, but in the context of preparing vinyl-end capped ethylene oligomers using Group 6 metallic complexes only.